1. Field of the Invention
This invention relates to an abrasive article comprising a lubricating particulate additive, and to methods for the manufacture and use of such an article. The article is useful as a polishing film for polishing the exposed ends of a fiber optic connector, for example.
2. Description of the Related Art
Fiber optic connectors of a wide variety of designs have been employed to terminate optical fiber cables and to facilitate the connection of the cables to other cables or other optical fiber transmission devices. A typical optic fiber connector includes a ferrule, which mounts and centers an optical fiber or fibers within the connector. The ferrule may be fabricated of ceramic materials.
A single mode fiber optical connector includes a glass core with an outer diameter of about 9 microns surrounded by a glass cladding with an outer diameter of about 125 microns (collectively the xe2x80x9cglass fiberxe2x80x9d). A ceramic ferrule surrounds the glass fiber. The ceramic ferrule has an outer diameter of about 2.0 millimeters at its outer ends and the diameter increases along a beveled edge (approximately 45xc2x0) to about 2.5 millimeters at its widest portion. During manufacture, the glass fiber is threaded through the ceramic ferrule and attached with an epoxy resin. The excess glass is then cleaved from the newly formed fiber optical connector, and the ends are polished to a fine finish.
A pair of fiber optic connectors or a connector and another optical fiber transmission device often are mated in an adapter which centers the fibers to provide good transmission. The adapter couples the connectors together so that their encapsulated fibers connect end-to-end to permit the transmission of light. The adapter may be an in-line component, or the adapter can be designed for mounting in an opening in a panel, backplane, circuit board or the like.
The polishing of the connectors is a necessary and important step. The light travels through the glass fiber across the junction to the next connector. In order to achieve a good crossover of the light, the ends must be highly polished, and the polished ends of the glass fiber and the ceramic ferrule preferably lay within a common planar or slightly convex surface. A planar or slightly convex surface with a radius of curvature of between about 10 millimeters and about 20 millimeters gives maximum glass fiber contact area with the glass fiber in the adjacent connector. With good transmission of light across the junction, the backreflection of the connection, a measure of the amount of light lost across the junction, will be minimized.
The causes of backreflection at the junction between two joined fiber optic connectors are many. Among the causes are microscopic imperfections on and just below the surfaces of the polished ends of the fiber optic connector fibers. These imperfections can range from surface scratches to subsurface fractures caused by the grinding and polishing process itself. Another cause of backreflection arises from the fact that the ends of the joined fiber optic connectors are pressed and held together within their connectors with a specified force, usually about two (2) pounds. This force acts to compress the glass material of the fibers at their ends, creating an increasing molecular density gradient and thus an increasing optical index of refraction as light approaches the junction. This is especially a problem if the glass fiber protrudes beyond the ceramic ferrule. The increased index of refraction in the region of the junction causes some of the light to be reflected back from the junction, resulting in backreflection. Another potential cause of backreflection is the presence of a space between the ends of two glass fibers if they are not completely flush with one another. This problem arises when the glass fiber is recessed within the ceramic ferrule. Even though polishing techniques have improved, manufacturers have been unable to overcome these and other causes of backreflection.
Generally, polishing films abrade different materials at different rates. In optical connectors, the glass fiber typically abrades at a rate faster than the harder ceramic ferrule. This results in an unacceptable undercut, where the glass fiber is abraded below the outer end surface of the ceramic ferrule after polishing. In order to achieve good cross over of light, the undercut is preferably about no more than 50 nanometers, and more preferably much less than 50 nanometers.
It is desirable to overcome the shortcomings of prior polishing articles and methods and to create a process that will polish fiber optic connectors to provide a high polish on the glass fiber and an essentially planar or slightly convex (radius of curvature of between about 10 millimeters and about 20 millimeters) end surface (e.g. with low undercut values). It is also desirable to provide an article for use in such a process and a process for the manufacture of such an article.
The present invention provides an abrasive article, which comprises a backing having a surface. The surface is covered with a coating formed of a binder, abrasive particles associated with the binder, and a lubricating particulate additive comprising polytetrafluoroethylene associated with the binder. The article is useful in the polishing of fiber optic connectors because the lubricating particulate additive allows the polishing rate of the softer glass fiber material to be slower than the polishing rate of the harder ceramic ferrule material. The different polishing rates allow both materials to be polished in the same step using the same abrasive article to provide an acceptable polished surface.
Another aspect of the invention is a method of polishing a fiber optic connector having a contact surface. The method comprises a pre-polishing step comprising contacting the fiber optic connector contact surface with a first abrasive article and relatively moving the fiber optic connector and the first abrasive article. The method additionally includes a polishing step involving contacting the fiber optic connector contact surface with a polishing abrasive article comprising a backing having a surface and a coating on the surface. The coating comprises a binder, abrasive particles associated with the binder, and a lubricating particulate additive associated with the binder. The next step in the method involves relatively moving the fiber optic connector and the polishing abrasive article. Optionally, an additional pre-polishing step may be performed between the pre-polishing step and the polishing step, wherein the fiber optic connector contact surface is contacted with a second abrasive article and relatively moved with respect to the second abrasive article, the second abrasive article being different from the first abrasive article.
A third aspect of the invention is a method of manufacturing an abrasive article comprising spreading a flowable coating solution on a backing and solidifying the coating solution to provide the abrasive article. The coating solution is formed of a binder, abrasive particles, and a lubricating particulate additive comprising polytetrafluoroethylene. The coating solution is solidified. The coating solution may be solidified by any method known in the art, such as exposure to heat in an oven for a specified dwell time.
Throughout this application, the following definitions apply unless otherwise defined in the specification:
xe2x80x9cLubricating particulate additivexe2x80x9d refers to a non-metallic material, which is substantially solid at room temperature.
xe2x80x9cWaxxe2x80x9d refers to an organic semi-crystalline solid.
xe2x80x9cProtrusionxe2x80x9d refers to the average distance between the glass fiber end surface and a virtual spherical surface fitted to a spherically polished ceramic ferrule if the glass fiber protrudes from the end surface of the ceramic ferrule. Protrusion is shown with a positive number.
xe2x80x9cUndercutxe2x80x9d refers to the average distance between the glass fiber end surface and a virtual spherical surface fitted to a spherically polished ceramic ferrule if the glass fiber is recessed within the ceramic ferrule. Undercut is shown with a negative number.
xe2x80x9cFlowablexe2x80x9d in reference to coating compositions herein, refers to material that can be spread across a surface using any of a variety of coating methods such as knife coating, for example.
xe2x80x9cBackreflectionxe2x80x9d refers to a measurement of the reflection of light at a planar junction of two materials having different refractive indices. As used herein, it is generally the measure of light reflection through the junction of two fiber optic connectors.
Backreflection is specified and measured in decibels (dB) and is calculated as follows:
10 log10(Preflected/Pin)
Where Preflected is the optical power reflected at the junction between two mated connectors and Pin is the optical power that enters the junction between the two connectors. Thus, a connector with a more negative backreflection transfers more signal from one cable to another and reflects less back as backreflection.